Which of the following is user-defined header file extension used in C++?

In C++, user-defined header files typically use the '.h' extension. This extension is standard for declaring functions, classes, and variables that are meant to be included in multiple source files. The '.hg' extension is used by the Mercurial version control system and is not related to C++ files. The '.cpp' extension is used for C++ source files, not header files. Therefore, only the '.h' extension is correct for user-defined header files in C++.